Pipetting method and pipetting device

ABSTRACT

A method is disclosed for controlling at least one of aspirating and of dispensing a liquid dose or of producing a liquid dose in a pipette or in receptacle, the method having the steps of first loading a first pipette with a first working medium at a first pressure having a first sign with respect to a reference pressure, thereby dispensing the liquid dose into the receptacle or aspirating the liquid dose into the first pipette, second loading a second pipette with a second working medium at a second pressure having a second sign with respect to the reference pressure, thereby dispensing the liquid dose into the receptacle or aspirating the liquid dose into the second pipette, discharging a pressure in the first working medium through a controlled valve arrangement to the reference pressure between first and second loadings. A pipetting device is also disclosed.

The invention addressed herein relates to a method of controlling atleast one of aspirating and of dispensing a liquid dose or of producinga liquid dose in a pipette or in receptacle. Under further aspects, theinvention relates to a pipetting device.

In the field of liquid handling, it is common practice to use pipettesto aspirate and dispense a liquid. Such a liquid may e.g. be a chemicalproduct or a sample of a bodily fluid. One type of pipetting devices isthe so-called air displacement pipette. When using this type of pipette,a defined volume of a working medium, in this case air, is loaded intothe pipette or removed from the pipette. Thereby a pressure on one sideof the liquid in a pipette is decreased or increased with respect toreference pressure, such that a force results, which drives the liquidout of the pipette or into the pipette. We understand throughout thepresent description and claims under “a pipette” a tubular member withone opening for aspiration and release of a liquid product dose and inaddition, with a second opening whereat aspirating with under-pressureor dispensing with over-pressure, respectively, is controllably applied.

In fields as for example pharmaceutical research, clinical diagnosticsand quality assurance, highly automated facilities for the handling,processing and analyzing of liquids are in use. In such facilities,pipetting devices often play a central role in producing liquid doses ofa predetermined amount and in transporting doses of liquid betweendifferent stations for processing or for analyzing the liquid. Accuracyand precision of the produced liquid doses is of large importance. Ingeneral, rapid processing is desired. This can be achieved by parallelhandling of liquid doses or by applying fast repetition rates.

From the publication WO 2010/084208 a pipetting arrangement is knownwhich comprises at least two sets of pipettes, whereby each set ofpipettes is operationally connected via a controllableON/OFF-(open/closed) valve to a common aspiration port. This way, atleast two sets of pipettes, can share one pumping arrangement, inparticular one precision pumping arrangement.

The object of the present invention is to provide an alternative methodof controlling at least one of aspirating and of dispensing a liquiddose or of producing a liquid dose in a pipette or in receptacle.

This object is achieved by a method according to claim 1.

The method according to the invention is a method of controlling atleast one of aspirating and of dispensing a liquid dose or of producinga liquid dose in a pipette or in receptacle.

The method comprises the step of first loading a first pipette with afirst working medium at a first pressure having a first sign withrespect to a reference pressure, wherein an amount of the first workingmedium flows across a flow restriction towards or away from the firstpipette, thereby dispensing the liquid dose into the receptacle oraspirating the liquid dose into the first pipette.

The method comprises the step of second loading a second pipette with asecond working medium at a second pressure having a second sign withrespect to the reference pressure, thereby dispensing the liquid doseinto the receptacle or aspirating the liquid dose into the secondpipette.

Further, the method comprises discharging a pressure in said firstworking medium through a controlled valve arrangement to the referencepressure between said first and said second loadings.

Surprisingly, the step of discharging a pressure in said first workingmedium through a controlled valve arrangement to the reference pressurehas the effect that the liquid doses produced by the method achieveaccurately the amount of liquid and the amount of liquid is highlyreproducible. Thereby, the reproducibility is not affected by thepreceding steps, such as the number of preceding aspirating ordispensing steps already performed with the same pipette or anotherpipette before starting the method for producing the liquid dose.Furthermore, the step of discharging a pressure in said first workingmedium through a controlled valve arrangement to the reference pressurehas the effect that the smallest volume that can be aspirated in acontrolled way is significantly lowered compared to a method withoutthis step of discharging.

Here, we use the term ‘loading’ to comprise loading positive pressure aswell as loading negative pressure (i.e. applying under-pressure orvacuum). The term ‘loading’ shall not imply a direction of flow of amedium. Stated differently, during the ‘loading’, working medium mayflow towards the pipette or away from the pipette.

The first and/or the second working medium may be a liquid or a gaseousfluid. In both cases, a container with a working medium having referencepressure may be provided for discharging a positive or negativepressure, i.e. for receiving excess working medium or providing workingmedium to be loaded to the pipette.

In one embodiment of the method according to the invention, which may becombined with any of the embodiments still to be addressed unless incontradiction, the working medium is a gaseous medium and/or thereference pressure is ambient pressure.

The first and/or the second gaseous medium may e.g. be air. It isconceivable as well, to use another gas, e.g. nitrogen, helium, argon,or another inert gas, if oxidation or other chemical reactions of theliquid to be handled with the gaseous medium are of concern. Thereference pressure may be ambient pressure. A working medium being agaseous medium may be brought directly in contact with ambient air fordischarging a pressure. The working medium may have undergone a particlefiltering step. Particularly in the case of a working medium being air,humidity of the working medium may be controlled.

In one embodiment of the method according to the invention, which may becombined with any of the embodiments still to be addressed unless incontradiction, there is at least one of the following valid:

a) said first and second loadings are equal;

b) the first and second loadings are different;

c) said first pipette is said second pipette;

d) said first and second pipettes are different pipettes

e) the absolute value of said first pressure is equal to the absolutevalue of said second pressure;

f) the absolute value of said first pressure is different from theabsolute value of said second pressure;

g) said first sign is equal to said second sign;

h) said first sign is opposite said second sign;

i) said first working medium is equal to said second working medium;

j) said first working medium is different from said second workingmedium.

Several loadings may be applied to the same pipette. The second loadingmay as well be applied to another pipette, e.g. another pipetteconnected to a manifold connecting a single pressure source with a setconsisting of several pipettes. In both cases, the second loading startswith more clearly defined pressure conditions, after the discharging ofpressure in the working medium. Different loadings with a working mediumat a first or second pressure may comprise loadings at positive andnegative pressure, as well as different absolute values of pressure.During a pipetting process a switch in composition of the working mediummay occur, e.g. from air being the first working medium to nitrogenbeing the second working medium. In this case, the step of discharging apressure in said first working medium through a controlled valvearrangement to the reference pressure accelerates the replacement of thefirst working medium. In this case, a sequence of loading anddischarging steps may be applied to quickly decrease the remainingoxygen level in the system.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, the methodcomprises establishing an intermediate time span between said first andsaid second loadings, in particular wherein said intermediate time spanis variable, and performing said discharging during a discharge timespan, whereby said discharge timespan is shorter than said intermediatetimespan, in particular wherein the step of discharging is executed as adischarge burst of the working medium.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, the pressureis applied in steps.

With this embodiment, any desired time course of the pressure in thefirst or second working medium is achievable.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, a pressurepulse having a sign inverse to the first sign is applied to the firstworking medium before discharging the pressure.

With this embodiment, a very fast return to reference pressure ispossible. Even a sign change in the pressure applied is possible,whereby a change in the direction of movement of the liquid can beenforced, e.g. for releasing a drop at the end of a dispensing step.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, a sequence ofthe first loading and the second loading is repeated, selectingsubsequent intermediate time spans to be equal or to vary.

The intermediate time span may be fixedly programmed into the timecontroller or the time span may be made dependent on conditionsoccurring during the pipetting process.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, the methodbeing a method for producing a liquid dose in a pipette, the methodcomprises the steps of

(a) dipping one of the first and second pipette's tip into a liquid,

(b) loading the one pipette with a respective working medium at arespective one of the first and second pressures with the respective oneof the first and second signs being negative, thereby aspirating theliquid dose into the pipette,

(c) confining the volume of the respective working medium.

The confining of the volume of the respective working medium at the endof the aspirating step has the effect that the liquid dose produced inthe pipette is held in the pipette, i.e. dripping out of the pipette isprevented.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, the methodbeing a method for producing N liquid doses in at most N receptacles,the method comprises the steps of

(a) providing a liquid in the first pipette;

(b) performing the first loading with a first sign being positivethereby dispensing a first liquid dose into a first receptacle;

(c) selecting said second pipette being the first pipette, the secondsign being equal to the first sign, the second working medium being thefirst working medium and performing the second loading, therebydispensing a further liquid dose into a further receptacle;andrepeating for N−1 times step (c), performing the discharging betweensubsequent loadings.

With this embodiment, a reliable method of multiple dispensing from asingle pipette is made available. The discharging steps between thesingle dispensing steps have the effect that the amount of liquid dosesproduced in each dispensing step is accurate and reproducible andindependent of the number of preceding dispensing steps.

Particularly in the case of aspirating once and then dispensing N equaldoses, it is an often observed fact that the volume of the first portionis different from the stable volume reached after a longer series ofdispensing steps. A known solution for this problem is to discard e.g.the first portion. Particularly in the case of low numbers of portions,such as 2 to 4 portions needed, the fraction of discarded liquid issignificant. The increased reproducibility achieved by the applicationof the discharging steps between subsequent loadings according to theembodiment of the method, i.e. in this case the application ofdischarging steps between subsequent dispensing steps of amulti-dispense sequence, reduces the need to discard the first portion,which consequently reduces the amount of wasted liquid and may improvethe efficiency of processes.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, the methodbeing a method for producing a liquid dose in a pipette, the methodcomprises the steps of

(a) dipping the first pipette's tip into a liquid,

(b) loading the first pipette with the first working medium at the firstpressure with the first sign being negative, thereby aspirating liquidinto the pipette,

(c) selecting the second pipette being the first pipette, the secondsign being equal to the first sign and performing the second loading.

With this embodiment, a method with succession of several aspiratingsteps is provided.

In another embodiment of the method according to the invention, whichmay be combined with any of the preaddressed embodiments and any of theembodiments still to be addressed unless in contradiction, the methodbeing a method for producing a liquid dose in a receptacle, the methodcomprises the steps of

(a) dipping the first pipette's tip into a liquid;

(b) loading the first pipette with a working medium at the firstpressure with the first sign being negative thereby aspirating theliquid dose into the pipette;

(c) moving the first pipette over a receptacle;

(d) selecting the second pipette being the first pipette and performingthe second loading with a second sign being positive thereby dispensingthe liquid dose into the receptacle.

This embodiment of the method provides a “suck-and-spit” method, i.e. amethod of producing a liquid dose by aspirating a liquid dose followedby dispensing the same liquid dose into a receptacle.

The pipetting device according to the invention is a pipetting devicefor a liquid driven by a working medium. The pipetting device comprisesat least one pipette connector adapted to releasably attach a pipette ata pipette side of the pipette connector. The at least one pipetteconnector has an opening towards the pipette side. To each pipetteconnector, a pipette may be attached when the pipetting device is inuse. The pipetting device comprises at least one positive and/ornegative pressure source. The pipetting device further comprises a flowconnection between the opening of the at least one pipette connector andthe at least one pressure source. The pipetting device comprises a flowrestriction, preferably a capillary, which is arranged in the flowconnection between the opening of the pipette connector and the pressuresource. The flow restriction separates the flow connection into anupstream portion and a downstream portion with respect to the flowrestriction. Further, the pipetting device has a controllable dischargevalve arrangement from the flow connection to reference pressure. Thepipetting device finally comprises a time controller. The timecontroller is operationally connected to the controllable dischargevalve arrangement. The operational connection allows to control thetiming of the opening and closing of the valves comprised in the valvearrangement. The controllable discharge valve arrangement provides thepossibility to discharge a pressure in a working medium to referencepressure. For use with a gaseous working medium the flow connection maybe built as a gas flow connection. The controllable discharge valvearrangement may be designed to selectively establish a connection fromthe flow connection to ambient, reference pressure being defined byambient pressure in this case.

A pipetting device with features as defined in the present invention hasthe advantage that it allows to aspirate small volumes in a reproducibleway. The minimum volume to be aspirated may be selected significantlylower than with a similar pipetting device not having a controllabledischarge valve arrangement. This advantage is important forsingle-pipetting of small volumes, such as volumes smaller than 5microliters. Surprisingly, variations of the minimum aspirateable volumeand variations in effectively aspirated volume dependent from individualvalves (i.e. from production tolerance of the valves) may be reduced aswell, in a pipetting device according to the present invention. Thisadvantage is particularly relevant in multi-channel pipetting devices,wherein each pipetting channel has its own set of valves.

With the pipetting device according to the present invention, a definedpressure may be applied at the beginning of a defined time span, wherebythe volume flow of the working medium is defined by the properties ofthe flow restriction and the pressure applied. Thus, the amount ofliquid aspirated into the pipette or dispensed from the pipette can becontrolled in very precise way by controlling the time span.

With the flow restriction being a capillary, the dependence of the flowrate on the pressure difference is very linear, which helps to achievehigh precision in the volume of the produced liquid doses.

The flow restriction may be e.g. a glass capillary, in particular afused silica capillary. The flow restriction may be built asmicro-channel, e.g. a laser structured micro-channel. The volume of thedownstream portion with respect to the flow restriction may be kept assmall as possible.

The pipette connector may be adapted to releasably attach a pipette,which is a so-called disposable tip, e.g. made of a plastic materialsuch as polypropylene. Such disposable tips may be plugged onto aconical section of the pipette connector. The pipette connector may beadapted to releasably attach a pipette, which ends in a hollow needlemade of metal, such as e.g. stainless steel, and which may be releasablyattached to the pipette connector by means of a swivel nut or a screw.

In an embodiment of the pipetting device according to the invention,which may be combined with any of the embodiments still to be addressedunless in contradiction, the time controller comprises a pulse generatorthe output thereof being operationally connected to a control input ofthe discharge valve arrangement.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, the controllable discharge valve arrangement comprises atwo-way valve and/or a switching valve, preferably a fast switchingvalve having a switching time in the millisecond range, in particular arotary valve.

A valve having a response time of less than 3 milliseconds may be usedin the controllable discharge valve arrangement. Response times ofapproximately 1.6 milliseconds may be achieved with directly switchingmagnet valves, which are normally closed by means of a spring, and whichare opened by applying a current to a coil of an electromagnet.Increased current may be applied just at the beginning of the opening ofthe valve in order to achieve short response times. This type of valveshas the further advantage that current flows only during the relativelyshort opening periods. This way power consumption as well as possiblydisturbing heating of the working medium is kept at a low level.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, the pipetting device comprises at least one of a positiveand of a negative pressure source with respect to reference pressure,preferably a respective pressure tank. The positive and the negativepressure sources may be constant pressure sources; in particular, theymay be built as pressure tank of large volume or may comprise means fornegative feedback control of the pressure.

With this embodiment, applying a positive or a negative pressure maysimply and very quickly be performed by opening a valve in a flowconnection leading to the respective pressure source.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, the pressure source comprises means for varying apressure delivered by the at least one pressure source.

Such means for varying a pressure may e.g. be a piston pump.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, at least a further controllable valve is operationallyinterconnected between the pipette connector and the at least onepressure source.

A further controllable valve e.g. allows to close the connection to theat least one pressure source while performing the discharging ofpressure to reference pressure. It further allows to close theconnection for a time span, when another pressure source, e.g. withpressure of opposite sign, is connected to the pipette.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, the pipetting device comprises a positive pressure sourceand a negative pressure source, wherein a first controllable valve isoperationally interconnected between the positive pressure source andthe pipette connector and a second controllable valve is operationallyinterconnected between the negative pressure source and the pipetteconnector.

This embodiment has the advantage that extremely fast changes of thepressure conditions in the flow connection to the pipette may beachieved. The opening time intervals of the first and the secondcontrollable valve may partially overlap, such that very fasttransitions from positive to negative pressure or from negative pressureto positive pressure may be achieved. The first and the secondcontrollable valves may be two-way valves. The first and the secondcontrollable valves may be arranged adjacent to a common section of theflow connection. Such a common section of the flow connection mayfurther be adjacent to the controllable discharge valve arrangement,which may be implemented as two-way valve, too. A common section of theflow connection may e.g. be in the upstream portion with respect to theflow restriction. Such a common section of the flow connection may e.g.have the form of a manifold with four arms, a first arm leading to thepositive pressure source, a second arm leading to the negative pressuresource, a third arm leading to the controllable discharge valvearrangement and a fourth arm leading to the flow restriction and furtherto the opening on the pipette side of the pipette connector. In such anarrangement, all three valves need to be switched with approximatelyequal frequency, such that all three valves have similar operatinglifetimes. The controllable discharge valve arrangement, the firstcontrollable valve and the second controllable valve may beoperationally connected to the same time controller. They may be threevalves of identical type. The valves may e.g. be direct switching magnetvalves, which are normally closed, and which are opened by applyingcurrent to a coil.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, the controllable discharge valve arrangement is connectedto at least one of the upstream portion of the flow connection and tothe downstream portion of the flow connection.

With a controllable discharge valve arrangement being connected to theupstream portion of the flow connection, the pressure applied to theupstream side can be quickly released by discharging the pressure toreference pressure. The pressure that directly acts onto the liquid inthe pipette may, depending on the dimension of the flow restriction andthe amount of working medium in the pipette, the pipette connector anddownstream portion of the flow connection, only decay slowly. In thiscase, a controllable discharge valve arrangement being connected to thedownstream portion of the flow connection, helps to faster discharge thepressure from the pipette side of the flow restriction to referencepressure. A combination of the two controllable discharge valvearrangements may be most effective, however at the price of anadditional valve.

Another embodiment of the pipetting device according to the invention,which may be combined with any of the preaddressed embodiments and anyof the embodiments still to be addressed unless in contradiction,comprises a multiplicity of pipette connectors, each pipette connectorbeing connected to a separate downstream portion of the flow connectionwith respect to a separate flow restriction.

With this embodiment it is possible to drive a multiple pipetting headwith a single common pressure source. Simultaneous pipetting of a row ofa well plate or of a complete well plate is possible with such a device.A pipetting head with, for example, rows of 4 or 8 pipette connectors orarrays of 24 (4×6), 96 (8×12), 384 (16×24) or 1536 (32×48) pipetteconnectors, preferably arranged corresponding to the positions of thewells of a standard well plate with the respective number of wells, ispossible. In such a configuration, the number of expensive and/orvoluminous parts such as piston pumps, valves and the like can be keptmuch lower than the number of pipette connectors, resulting in lowerproduction cost and/or in a more compact design. With increasing numberof pipette connectors, the total volume of the flow connectionincreases. In a pipetting head with a large number of pipetteconnectors, accuracy and reproducibility of the liquid volumes createdbenefit strongly from the effect of the discharge valve arrangement, asremaining pressure in the flow connection can be discharged efficiently.With this embodiment, if one pipette fails, e.g. because air isaspirated, or a pipette is clogged, due to the presence of the flowrestriction in the branch of the failing pipette, this has only minoreffects on the pressure conditions for all the other pipettes.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, the upstream portion of the flow connection comprises ata multiplicity of branches and/or a buffer tank.

This embodiment is particularly suited for a pipetting head with a largenumber of pipette connectors. In this embodiment, too, accuracy andreproducibility of the liquid volumes created benefit strongly from theeffect of the discharge valve arrangement, as remaining pressure in theflow connection can be discharged efficiently also in the case when theflow connection is formed by a voluminous and/or complex manifold havinga buffer tank and/or a multiplicity of branches. With this embodimentpressure conditions in the upstream portion as well as the total flowresistance from a pressure source to an individual pipette connector maybe arranged to be the same for all pipette connectors in a simple way.

In another embodiment of the pipetting device according to theinvention, which may be combined with any of the preaddressedembodiments and any of the embodiments still to be addressed unless incontradiction, the downstream portion of the flow connection is built asa flexible tube with an inner diameter smaller than or equal 1 mm.

This embodiment allows to have flow connection between a movablepipetting head with at least one pipette connector and a set of valvesand flow restriction, which can be mounted in a fix position on thepipetting device. The small diameter of the flexible tube keeps the deadvolume in the downstream portion of the flow connection low. A flexibletube with a length of up to 1 m may e.g. be possible in this embodiment,while still achieving high precision with regard to pipetted volumes. Amulti-channel pipetting device having such a flexible tube with smallinner diameter for each channel is conceivable as well. The downstreamportion of the flow connection may be built as a flexible tube with aninner diameter as small as 0.75 mm or 0.5 mm.

The invention is further directed to a method of controlling at leastone of aspirating and of dispensing a liquid dose or of producing aliquid dose in a pipette or in receptacle according to the method aspectof the invention by means of a pipetting device according to the deviceaspect of the invention.

The invention shall now be further exemplified with the help of figures.The figures show:

FIGS. 1.a) to 1.d) show a schematic view of a pipetting device accordingto the invention in different states, while performing the methodaccording to the invention;

FIG. 2 a schematic view of a pipetting device according to anembodiment;

FIG. 3 a schematic view of a pipetting device according to anembodiment;

FIGS. 4.a) to 4.c) show schematic views of the variants of an embodimentof a pipetting device having the flow restriction at differentpositions;

FIG. 5 a schematic view of a pipetting device according to anembodiment;

FIG. 6 a schematic view of a pipetting device according to anembodiment;

FIG. 7 a schematic view of a pipetting device according to anembodiment;

FIG. 8 schematically a course of a pressure in a pipette over timeduring the steps according to an embodiment of the method according tothe invention;

FIG. 9 a schematic view of a pipetting device according to anembodiment;

FIGS. 10.a) to 10.b) show a comparison of two different time courses ofmeasured pressure during aspirating;

FIG. 11 a schematic view of pipetting device according to an embodiment.

FIG. 1.a) shows schematically and simplified, a pipetting deviceaccording to the invention. The pipetting device here is shown in astate ready to be used to produce a liquid dose in a receptacle 24. Aliquid 21 is provided in a pipette 16 placed over the receptacle. Aworking medium 22 fills a part of the pipette that is in fluidconnection with a controllable discharge valve arrangement 14, which ishere designed as a rotational valve. The rotational valve has aconnecting part that is rotatable as indicated by an arrow. The timecontroller 15 controls the position of this connecting part. The timecontroller is not shown anymore in FIGS. 1.b) to 1.d). The connectingpart is currently in a position that is closed, i.e. the working mediumis confined. The rotational valve can be switched to establish fluidconnection either to a pressure source 11 or to reference pressure 20.The pipette connector 13 has an opening 17 towards the pipette side ofthe pipette connector. A flow restriction 40 is arranged in the flowconnection 12. The flow restriction 40 separates the flow connection 12into an upstream portion 41 and a downstream portion 42 with respect tothe flow restriction.

FIG. 1.b) shows the pipetting device after the rotational valve has beenswitched to establish fluid connection between the pressure source 11and the pipette 16. Thereby, the pipette is loaded with a working medium22 at a pressure having in this case a positive sign with respect toreference pressure. An amount of the working medium flows across theflow restriction 40 towards the pipette. This way, a drop of liquid ispushed out of the tip of the pipette.

FIG. 1.c) shows the pipetting device after the rotational valve has beenswitched to establish fluid connection from the working medium toreference pressure 20, thereby discharging a pressure in the workingmedium through the controlled valve arrangement to reference pressure.

FIG. 1.d) shows the pipetting device after the rotational valve has beenswitched again to establish fluid connection between the pressure source11 and the pipette 16 leading to a second loading of the pipette with aworking medium. In this case again, the pressure has a positive signwith respect to reference pressure, dispensing a second liquid dose intoa second receptacle 24. Again, an amount of the working medium flowsacross the flow restriction 40 towards the pipette.

FIG. 2 shows a schematic view of a pipetting device according to anembodiment comprising one pipette 21. The pipette is attached to apipette connector 13. A flow connection 12 provides a connection betweenthe pipette connector 13 and a pressure source 11. In the flowconnection 12 a flow restriction 40 is arranged between the pipetteconnector 13 and the pressure source 11. A working medium 22 is loadedinto the pipette at a pressure determined by the pressure source. Acontrollable discharge valve arrangement 14 is formed by a two-way valveprovided in an arm branching of the flow connection 12 and having an endportion in contact with reference pressure. The controllable dischargevalve arrangement 14 is connected to the upstream portion 41 withrespect to the flow restriction. A time controller 15 is operationallyconnected to the controllable discharge valve arrangement.

FIG. 3 shows a schematic view of a pipetting device according to anembodiment comprising a positive pressure source 11′ and a negativepressure source 11″, each being built as a pressure tank. The positiveand the negative pressure sources may be constant pressure sources; inparticular, they may be built as pressure tank of large volume or maycomprise means for negative feedback control of the pressure. Arotational valve allows to selectively connect the pipette connector 13to the positive pressure source 11′, to the negative pressure source 11″or to reference pressure 20, as well as to confine the working medium inthe flow connection 12 by bringing the rotational valve into a closedposition. Several pipettes 16, in this case three pipettes, share acommon flow connection 12 designed as a manifold having several arms. Inevery arm connecting a single pipette, a two-way valve is provided. Eachof these two-way valves allows to selectively connect or disconnect therespective pipette to the rotational valve. A flow restriction 40 isarranged in the flow connection 12. More specifically, the flowrestriction 40 is arranged in the section between the valve 14 and thepoint, where the flow restriction branches out into separate armsleading to the individual pipettes. I.e. the separate arms are in thedownstream portion 42 with respect to the flow restriction. The timecontroller 15 is operatively connected to the two-way valves as well asto the rotational valve. This arrangement allows to selectivelydischarge pressure from a single pipette as well as from a set ofpipettes or, in particular, of all pipettes together. This embodiment ofthe pipetting device may comprise a larger number of individuallycontrollable pipetting channels, e.g. with 8 or 16 pipettes in a row, orarrays of 24, 96, etc. pipettes.

FIGS. 4.a) to 4.c) show schematic views of variants of an embodiment ofa pipetting device comprising a flow restriction 40. In each variant,the flow restriction separates the flow connection 12 into an upstreamportion 41 and a downstream portion 42 with respect to the flowrestriction. In the variant shown in FIG. 4.a), an arm of the flowconnection 12 branches of the upstream portion. A two-way valve isarranged in this arm. The two-way valve is operationally connected to atime controller 15, thus forming a controllable discharge valvearrangement 14 being connected to the upstream portion 41 of the flowconnection 12. In the variant shown in FIG. 4.b), the controllabledischarge valve arrangement 14 is connected to the downstream portion 42of the flow connection 12. The variant shown in FIG. 4.c) comprises twotwo-way valves being controlled by the time controller 15, each fordischarging the upstream portion 41 and the downstream portion 42 of theflow connection, respectively.

FIG. 5 shows a schematic view of a pipetting device according to anembodiment comprising a positive pressure source 11′ and a negativepressure source 11″, each of which is built as pressure tank. A flowconnection 12 to a pipette connector 13 branches into two arms, oneleading to the positive pressure source, the other leading to thenegative pressure source. A two-way valve 51 and a two-way valve 52 areprovided in each of the two arms. A third valve, being a switching valve53 allows to selectively connect the first arm of the flow connection toeither the positive pressure source 11′ or to reference pressure 20. Allthree valves 51, 52, 53 mentioned above are operatively connected to atime controller 15, as indicated by dashed lines. The first two-wayvalve 51 and the switching valve 53 in combination form a controllabledischarge valve arrangement 14. A flow restriction 40 is arranged in theflow connection 12.

FIG. 6 shows a schematic view of a pipetting device according to anembodiment similar to the embodiment shown in FIG. 5 also comprising apositive and a negative pressure source. The connection between thepipette connector 13 and each of the two pressure sources can be openedand closed individually by the two-way valves 51 and 52. In thisembodiment, however, the controllable discharge valve arrangement 14consists of a two-way valve that is arranged in a direct connection fromthe flow connection 12 to reference pressure. A flow restriction 40 isarranged in the flow connection 12. More specifically, the flowrestriction 40 is arranged in the section of the flow connection betweenthe branching off towards the controllable discharge valve arrangement14 and the pipette connector 13. A pressure sensor 91 is on one side influid connection to the upstream portion with respect to the flowrestriction and on its other side in in fluid connection to thedownstream portion with respect to the flow restriction. The pressuredifference measured by pressure sensor 91 is an indication of the amountof working fluid passing across the flow restriction. A further pressuresensor 92 measures the pressure difference between the downstreamportion and a reference pressure, e.g. atmosphere pressure.

FIG. 7 shows a schematic view of a pipetting device according to anembodiment comprising a pressure source 11 having means for varying thepressure delivered by the pressure source. The means for varying thepressure here is implemented as a piston pump. The flow connection 12from the pressure source to the pipette connector 13 can be opened orclosed by means of a first two-way valve 51 provided in the flowconnection 12. A second two-way valve is arranged in a connection fromthe flow connection 12 to reference pressure 20. This second two-wayvalve is operatively connected to a time controller 15, thus forming acontrollable discharge valve arrangement 14. The time controller 15 isfurther operatively connected to the first two-way valve and the pistonpump in order to control time sequences of loading the pipette connector13 as well as a pipette attached thereto with a working medium 22 havingat a predetermined pressure. A flow restriction 40 is arranged in theflow connection 12. Pressure sensors 91 and 92 are arranged similar tothe arrangement shown in FIG. 6.

FIG. 8 shows a possible course of a pressure in the pipette connectorside of a pipette over time t during the steps according to anembodiment of the method according to the invention. The pressure courseshown here is the pressure course in two successive dispensing stepsfrom the same pipette. At the beginning, during time span 80, a liquidis already provided in the pipette, the pressure corresponds to thepressure being marked as 0 on the pressure axis p, correspondingapproximately to reference pressure. During a first loading of thepipette with a working medium at a pressure above reference pressure,pressure in the pipette rises and liquid is dispensed from the pipette.This first loading occurs in time span 81. By applying a negativepressure pulse in time span 82, the dispensing is stopped quickly, andthe release of a drop formed at the tip of the pipette is ensured. Thenegative pressure pulse has the side effect that pressure in the pipettefalls below reference pressure. The following discharging step duringtime span 83, whereby the pressure of working medium is dischargedthrough a controlled valve arrangement to reference pressure, leads to areturn of the pressure in the pipette to approximately referencepressure. In fact, after the dispensing step, the pressure measured bypressure sensor 92 is slightly higher than before the dispensing step,as the amount of liquid in the pipette is reduced, thus leading to achange in hydrostatic pressure due to the reduced height of the liquidcolumn present in the pipette. After an additional waiting time span 84a second loading of the pipette with the working medium at a pressureabove reference pressure is performed. In this case, the second pipetteis identical to the first pipette and the second working medium isidentical to the first working medium. The time spans 83 and 84 togetherform an intermediate time span between the first and second loadings.During the second loading, again a dispensing of a liquid dose takesplace. Pressure plateaus reached in intervals 81 and 85 are the same, incase of an ideal pressure reservoir for the positive pressure source.This may be achieved by selecting a reservoir having a large volume inrelation to the volumes dispensed. For comparison, a pressure course indashed line is shown for a similar process, but without a dischargingstep during the time span 83. In the case of the pressure course withouta discharching step, the pressure drifts over a longer time and in anuncontrollable way towards a stable pressure value, whereas in the case,when a discharching step is applied, a stable pressure value isestablished shortly after the end of time period 82. It is clearlyvisible that shortening the time 84 would lead to different conditionsfor the second dispensing step, in case that no discharging step isperformed. Thus, the method according to the invention improves thereproducibility of pipetting results.

FIG. 9 shows a schematic view of a pipetting device according to anembodiment combining features of the embodiment shown in FIG. 5 incombination with pressure sensors 91 and 92. A controllable dischargevalve arrangement 14 is connected to the upstream portion 41 of the flowconnection 12. The controllable discharge valve arrangement 14 is formedby the combination of the two-way valve 51 and the switch valve 53. Apressure sensor 91 is arranged to measure a pressure difference betweenthe upstream portion 41 and the downstream portion 42 of the flowconnection, i.e. the pressure difference occurring over the flowrestriction. This pressure difference is proportional to the flow ofworking medium through the flow restriction. A further pressure sensor92 is arranged to measure a pressure difference between the downstreamportion 42 of the flow connection and reference pressure 20. Signalsfrom the two pressure sensors 91 and 92 may be used to increase theprecision of the doses produced and to detected malfunctions in thepipetting process.

FIGS. 10.a) and 10.b) show time courses of the pressure measured in apipetting device as shown e.g. in FIG. 6 or in FIG. 9 during anaspirating process. Time t is shown on a millisecond timescale andpressure p is in arbitrary units with an arbitrary offset. The pressurecurves 101 and 103 shown as solid lines are pressure curves measured bythe sensor 91, thus indicating a flow of the working medium through thecapillary and correspondingly a flow of liquid into the pipette. Thesign of the pressure axis p is such that the positive directioncorresponds to a flow out of the pipette. The pressure curves 102 and104 shown as dashed lines are pressure curves measured by the sensor 92.FIG. 10.a) shows the time course of the pressure without the step ofdischarging a pressure in the first working medium to referencepressure, whereas FIG. 10.b) shows the time course of the pressure withthe step of discharging a pressure in the first working medium through acontrolled valve arrangement to reference pressure, the dischargingbeing applied starting at time 60 milliseconds, but otherwise with thesame parameters as in FIG. 10.a). The start of discharging pressure toreference pressure is marked by reference sign 106. The comparison ofthe two pressure curves 101 and 103 clearly shows that the flow ofliquid and working medium is stopped quicker and at a defined momenttime, if the step of discharging pressure is applied. The area 105marked by diagonal hatching corresponds to a time integral of flow, i.e.an aspirated volume. If the step of discharging the pressure in thefirst working medium to reference pressure is not applied, this volume,the size of which depends on the ‘dead volume’ between capillary andupstream valves, is unavoidably aspirated, i.e. it defines the minimumvolume for single pipetting. With the timing as shown in FIG. 10.a), theaspirated volume corresponding to the area 105 is about half of thevolume aspirated in total. By applying the step of discharging thepressure in the first working medium through a controlled valvearrangement to reference pressure, however, it becomes possible toaspirate much smaller volumes than this volume in a controlled way, ascan be seen in FIG. 10.b). The pressure curve 103 displays a nearlyrectangular form—here in the time between 0 and 60 milliseconds, whichmeans, that the aspirated volume depends linearly on the aspirating timeinterval. As an example, without using a controllable discharge valvearrangement, minimal aspirated volumes in the range of 10 microlitersmay be achieved. By discharging a pressure in the working medium througha controlled valve arrangement according to a step of the presentinvention, this minimal aspirated volume may be brought down to 0.1microliter. The effective numbers may depend on the pressures in thepressure sources and on the dead volumes in the flow connection.

A similar effect is achieved for dispensing.

An example with measured quantitative results is given in the followingtables, Table 1 and Table 2. Aspirating steps similar to the onesdisplayed in FIG. 10.a) and FIG. 10.b) respectively have been performedon a pipetting device, wherein a dead volume in the downstream portionwith respect to the flow restriction is defined by a tube having aninner diameter of 0.5 millimeter and a length of 1 meter. A disposabletip having a volume of 200 microliters has been used. The negativepressure source was held at a pressure of 250 mbar, i.e. at about 750mbar below reference pressure. Opening times of the valve to achieveseven different aspirating times (see column “AspTime”) 30 ms, 20 ms, 18ms, 16 ms, 14 ms, 12 ms and 10 ms have been applied. Each aspiratingtime has been used in four pipetting steps with a discharge stepaccording to the invention (Table 1), i.e. similar to FIG. 10.b), and infour pipetting steps without a discharging step (Table 2), i.e. similarto FIG. 10.a). Effectively aspirated volumes have been measuredgravimetrically.

TABLE 1 (with discharging step) AspVol with AspTime Discharge Valve Mean[ms] [μl] [μl] 30 6.99 6.94 6.95 6.96 6.96 20 1.9 1.9 1.95 1.91 1.92 181.54 1.54 1.54 1.52 1.54 16 1.15 1.17 1.15 1.18 1.16 14 0.7 0.74 0.730.72 0.72 12 0.18 0.17 0.17 0.32 0.21 10 0.1 0.08 0.07 0.08 0.08

TABLE 2 (without discharging step) AspTime AspVol no Discharge ValveMean [ms] [μl] [μl] 30 16.83 16.79 16.81 16.84 16.82 20 12.4 12.37 12.3912.22 12.35 18 11.66 11.83 12.06 11.94 11.87 16 11.5 11.88 11.57 11.4811.61 14 11.35 11.6 11.47 11.32 11.44 12 10.76 11.01 10.69 10.87 10.8310 10.76 10.62 10.6 10.69 10.67

Comparison of Table 1 and Table 2 shows that the minimum volume that maybe aspirated is drastically reduced from about 10 microliters achievablewithout applying a discharging step to about 0.08 microliters achievablewith applying the discharging step.

FIG. 11 shows a further embodiment of the pipetting device. Here theflow connection 12 is built as a manifold that has a multiplicity ofbranches 111, 112, 113, 114. Each branch connects a pipette connector 13to a common pressure source. Each branch leads through a flowrestriction of its own, such that each of a multiplicity of pipetteconnectors is connected to a separate downstream portion of the flowconnection with respect to a separate flow restriction. A simultaneousdispensing with all four pipette tips connected to the pipetteconnectors into four wells 24 is shown as an example of possible use ofthis embodiment. Each branch is equipped with pressure sensors 91, 92arranged around the flow restriction, as already discussed in thecontext of FIG. 9. In this configuration, the pressure sensors may beused to detect clogging of individual pipettes, erroneous aspiration ofair or other types of malfunction of an individual pipette tip. Theconnection between the pipette connector 13 and a positive pressuresource 11′ and a negative pressure source 11″ can be opened and closedindividually by the two-way valves 51 and 52. The controllable dischargevalve arrangement 14 comprises a two-way valve that is arranged in adirect connection from the flow connection 12 to reference pressure 20,which in this case is given by the pressure in the environment of thepipetting device. The three two-way valves are controlled by the timecontroller 15. The flow connection is built as a manifold that branchesout tree-like on two hierarchical levels, doubling the number ofbranches on each level. The individual branches are arranged such thatthe travel distance of the working medium from a central distributionport 110 to each flow restriction 40 is equal for all branches. Thisway, the timing of the pipetting process is not disturbed by distancedependent delays on individual branches.

A structure of branches similar to the one shown in FIG. 11 and havingequal travel distance on each branch may e.g. constructed for a 4×4array of pipette tips by a first four-fold branching leading to thecenters of 2×2 sub-arrays followed by a second four-fold branchingconnecting the individual flow restrictions. Six 4×4 block of this typemay be combined to a 96-pipette head with an array of 8×12 pipetteconnectors arranged corresponding to the positions of the wells of a96-well-plate. If each 4×4 block uses three two-way valve arranged asschematically shown in FIG. 11, a 96-pipette head may be built with only18 controllable two-way valves.

In the following, a workflow for the valve operations in an embodimentof the method is explained, whereby a pipetting device according to theembodiment as shown in FIG. 5 is used for performing the method. Theworkflow describes an aspirate step followed by a dispense step. In thedispense step a negative pressure pulse is used for drop release.

Aspirate, performed with pipette tip in liquid:

-   -   switch valve 53 to reference pressure (e.g. atmospheric        pressure);    -   open valve 52, starts aspirating liquid into the pipette;    -   close valve 52;    -   open valve 51 for 5 milliseconds, this leads to discharging the        negative pressure and stops aspirate;    -   close valve 51, now the working medium above the liquid in the        pipette is confined, such that the liquid is held in the        pipette.

In order to quickly stop aspirating, the step of closing valve 52 may aswell fall into the opening period of valve 51, such that discharging thenegative pressure very sharply defines the end of aspirating and definesthe pressure state in the pipette connector, too.

Dispense, performed with pipette tip over receptacle:

-   -   switch valve 53 to positive pressure source;    -   open valve 51, starts dispensing liquid;    -   close valve 51;    -   open valve 52, drop release by drawing liquid backward;    -   close valve 52;    -   switch valve 53 to reference pressure;    -   open valve 51, discharges remaining pressure to reference        pressure.

In order to achieve steep flanks in the time course of the pressure inthe pipette connector and fast switch over times, the step of closingthe valve 52 may as well be performed at the end of the above sequence,i.e. shortly after opening the valve 51 to discharge remaining pressureto reference pressure 20.

LIST OF REFERENCE SIGNS

-   10 pipetting device-   11 pressure source-   11′, 11″ positive/negative pressure source-   12 flow connection-   13 pipette connector-   14 controllable discharge valve arrangement-   14′ further valve-   15 time controller-   16 pipette-   17 opening of pipette connector (at pipette side)-   20 reference pressure-   21 liquid-   22 working medium-   23 liquid dose-   24 receptacle-   40 flow restriction-   41 upstream portion (of flow connection)-   42 downstream portion (of flow connection)-   51 two-way valve-   52 two-way valve-   53 switching valve-   80 time span before first loading-   81 time span during first loading-   82 time span during negative pressure pulse-   83 time span during discharging of pressure to reference pressure-   84 time span of waiting-   85 time span during second loading-   91 pressure sensor (difference over capillary)-   92 pressure sensor (difference to reference pressure)-   101 pressure curve (measured during aspirating)-   102 pressure curve (measured during aspirating)-   103 pressure curve (measured during aspirating)-   104 pressure curve (measured during aspirating)-   105 area (corresponding to an aspirated volume)-   106 start of discharging pressure to reference pressure-   110 central distribution port-   111, 112, 113, 114 branches of the flow connection-   P pressure-   P⁺ positive pressure-   P⁻ negative pressure-   t time

The invention claimed is:
 1. A method of controlling at least one ofaspirating and of dispensing a liquid dose (23) or of producing a liquiddose in a pipette or in a receptacle (24), the method comprising thesteps of first loading a first pipette (16) with a first working medium(22) at a first pressure having a first sign with respect to a referencepressure (20), wherein an amount of said first working medium flowsacross a flow restriction (40) towards or away from said first pipette,thereby dispensing the liquid dose into the receptacle or aspirating theliquid dose into the first pipette, second loading a second pipette witha second working medium at a second pressure having a second sign withrespect to said reference pressure, thereby dispensing the liquid doseinto the receptacle or aspirating the liquid dose into the secondpipette, discharging a pressure in said first working medium through acontrolled valve arrangement (14) to said reference pressure betweensaid first and said second loadings, the method comprising establishingan intermediate time span between said first and said second loadingsand performing said discharging during a discharge time span, wherebysaid discharge timespan is shorter than said intermediate timespan. 2.The method of claim 1, wherein the working medium is a gaseous mediumand/or said reference pressure is ambient pressure.
 3. The methodaccording to claim 1, wherein there is at least one of the followingvalid: a) said first and second loadings are equal; b) the first andsecond loadings are different; c) said first pipette is said secondpipette; d) said first and second pipettes are different pipettes e) theabsolute value of said first pressure is equal to the absolute value ofsaid second pressure; f) the absolute value of said first pressure isdifferent from the absolute value of said second pressure; g) said firstsign is equal to said second sign; h) said first sign is opposite saidsecond sign; i) said first working medium is equal to said secondworking medium; j) said first working medium is different from saidsecond working medium.
 4. The method according to claim 1, wherein thepressure that is discharged is applied in steps.
 5. The method accordingto claim 1, wherein a pressure pulse having a sign inverse to said firstsign is applied to said first working medium before discharging saidpressure.
 6. The method according to claim 1, repeating a sequence ofsaid first loading and said second loading, selecting subsequentintermediate time spans to be equal or to vary.
 7. The method accordingto claim 1 for producing a liquid dose in a pipette, the methodcomprising the steps of (a) dipping one of said first and secondpipette's tip into a liquid, (b) loading said one pipette with arespective working medium at a respective one of said first and secondpressures with the respective one of said first and second signs beingnegative, thereby aspirating the liquid dose into the pipette, (c)confining the volume of said respective working medium.
 8. The methodaccording to claim 1 for producing N liquid doses in at most Nreceptacles, the method comprising the steps of (a) providing a liquidin said first pipette; (b) performing said first loading with a firstsign being positive thereby dispensing a first liquid dose into a firstreceptacle; (c) selecting said second pipette being said first pipette,wherein said first pipette and second pipette are one in the samephysical object, said second sign being equal to said first sign, saidsecond working medium being said first working medium and performingsaid second loading, thereby dispensing a further liquid dose into afurther receptacle; and repeating for N−1 times step (c), performingsaid discharging between subsequent loadings.
 9. The method according toclaim 1 for producing a liquid dose in a pipette, the method comprisingthe steps of (a) dipping said first pipette's tip into a liquid, (b)loading said first pipette with said first working medium at said firstpressure with said first sign being negative, thereby aspirating liquidinto the pipette, (c) selecting said second pipette being said firstpipette, wherein said first pipette and second pipette are one in thesame physical object, said second sign being equal to said first signand performing said second loading.
 10. The method according to claim 1for producing a liquid dose in a receptacle, the method comprising thesteps of (a) dipping said first pipette's tip into a liquid; (b) loadingsaid first pipette with a working medium at said first pressure with thefirst sign being negative thereby aspirating the liquid dose into thepipette; (c) moving the first pipette over a receptacle; (d) selectingsaid second pipette being said first pipette, wherein said first pipetteand second pipette are one in the same physical object, and performingsaid second loading with a second sign being positive thereby dispensingsaid liquid dose into said receptacle.
 11. A method of controlling atleast one of aspirating and of dispensing a liquid dose or of producinga liquid dose in a pipette or in a receptacle according to claim 1 bymeans of a pipetting device.
 12. The method according to claim 1,wherein said intermediate time span is variable.
 13. The methodaccording to claim 1, wherein the step of discharging is executed as adischarge burst of the working medium.
 14. A method of controlling atleast one of aspirating and of dispensing a liquid dose (23) or ofproducing a liquid dose in a pipette or in a receptacle (24), the methodcomprising the steps of first loading a first pipette (16) with a firstworking medium (22) at a first pressure having a first sign with respectto a reference pressure (20), wherein an amount of said first workingmedium flows across a flow restriction (40) towards or away from saidfirst pipette, thereby dispensing the liquid dose into the receptacle oraspirating the liquid dose into the first pipette, second loading asecond pipette with a second working medium at a second pressure havinga second sign with respect to said reference pressure, therebydispensing the liquid dose into the receptacle or aspirating the liquiddose into the second pipette, discharging a pressure in said firstworking medium through a controlled valve arrangement (14) to saidreference pressure between said first and said second loadings, whereina pressure pulse having a sign inverse to said first sign is applied tosaid first working medium before discharging said pressure.